In German and English tool-steels a com bined percentage not exceeding 0.06 per cent of sulphur, phosphorus and copper is considered first class, while in America this combined per centage is about one-half as much and would not be expected to exceed 0.04 per cent in the best product.
Swedish bar or puddled bar may be and is melted directly or it may be first carburized by the cementation process. Cemented bar is used more largely abroad than in America. For the purpose of adding carbon, when the cemented bar is not used, it is customary to use either a pure Swedish charcoal pig iron or the American product known as °washed-metal.° Roth carry about 4 per cent carbon and are low in sulphur and phosphorus, but the former carries rather variable quantities of silicon and manganese, from which washed metal is nearly free. This is an advantage in favor of the Swedish pig, if the silicon and manganese can be maintained fairly uniform and not excessive in amount. In some cases all the carburizing in the crucible charge is effected by means of charcoal. Small quantities of oxide of man ganese or ferromanganese are usually em ployed, and for special steels metallic nickel, manganese, tungsten, chromium, or molyb denum or their ferroalloys are employed.
Crucibles.—In England, clay crucibles are almost exclusively used. They are nearly al ways made in connection with the works using them, the mixing usually being done by men who tread the clay with their bare feet. Little or no machinery is used in their production. The mixed ingredients are made into balls of suitable weight and these are dropped into a mold and a plug shaped like the inside of a crucible is driven in, forcing the clay up into the mold and around the plug, which is then withdrawn and the crucible removed from the mold and dried for several weeks. Before using, it is °annealed° or baked and is always filled while hot. The crucibles last only about three heats, carrying 60 pounds at the first heat and a less amount on each subsequent charge. This is because the metal attacks and weakens the crucible at the slag line and hence the level of the metal is lowered in the pot on each suc ceeding heat.
Graphite crucibles are generally used in Ger many, Austria and the United States. These according to analysis, quoted by Howe, may contain from 20 to 83 per cent of graphite, about 50 per cent being commonly used in America.
The best Ceylon graphite should be used. Arti ficial graphite made in electrical furnaces is unsuitable for crucibles because of its granu lar rather than lamellar structure. With the graphite is used either clay or a mixture of clay and old crucibles finely ground. It is also cus tomary to use a little coke dust in making clay crucibles. Graphite crucibles are much larger than clay ones and will carry the customary 90 to 100-pound charge from 5 to 10 heats. They will stand much abuse, especially in the way of sudden changes of temperature, and need not be filled hot each time. Indeed, it is quite cus tomary to allow them to cool off completely between heats, when they are cleaned and in spected. Graphite crucibles cost from five to eight times as much as clay crucibles, but this disadvantage is more than offset by their greater endurance and the much greater produc tion per crucible. The disadvantage frequently mentioned, that they impart variable amounts of carbon and silicon to the steel, is more fancied than real.
Furnaces and Fuel.— Several types of fur naces have been employed for crucible steel melting, but such steel for tool purposes is nearly always melted in shaft furnaces using coke as fuel or in Siemens regenerative fur naces using gaseous fuel,— either producer or natural gas. The latter is almost exclusively used in the Pittsburgh district and seems to be the ideal fuel because of its uniformity of com position and its freedom from sulphur. In ad dition to these fuels, anthracite has been employed to a limited extent in a special form of shaft furnace, somewhat resembling the coke-holes or furnaces. They differ in this, however, that they require forced draft and to provide this the ash-pit is enclosed and air is driven into it and thence through the bed of anthracite on the grate by means of a fan blower. Waste heat from anthracite furnaces may be used fortheating steam boilers. Petro leum has been used for fuel in the Nobel fur nace for melting crucible steel for castings, as in the Mitis process. We now hear little of anthracite or petroleum for crucible steel melt ing, and the writer is not aware that either of these fuels is actually being used in America at the present time.